A hydraulic operating system for a small vehicle including a bicycle is basically provided with a base member, a piston, a hydraulic fluid chamber, a diaphragm and a deformation adjustment structure. The base member includes a cylinder bore. The piston is movably disposed in the cylinder bore. The hydraulic fluid chamber is fluidly connected to the cylinder bore. The diaphragm is at least partially defining the hydraulic fluid chamber. The diaphragm is deformed by fluid pressure in response to movement of the piston in the cylinder bore. The deformation adjustment structure is configured to selectively change a deformation condition of the diaphragm between at least two different deformation conditions.